The ‘Dark Bronchus’ Sign: For diagnosis of PCP

Today I will discuss the importance of the ‘dark bronchus’ sign in
the diagnosis of uniform, diffuse ground glass opacification on high resolution
computerized tomography (HRCT). This sign is useful to identify diffuse ground
glass opacity on HRCT in cases of Pneumocystis carinii pneumonia who may
present with a normal or equivocal chest radiograph in the early course of
disease.

Chest radiograph is the initial investigation in HIV patients with
chest symptoms. But even in patients with proven PCP, radiographic findings may
be normal in up to 20-40%. Low incidence of PCP in patients with normal or
equivocal findings on chest radiograph despite high clinical suspicion
emphasizes the need for a noninvasive and widely available investigation in
such cases.

Various modalities to investigate symptomatic HIV patients with
normal, equivocal or nonspecific radiographic findings include carbon monoxide
diffusion in lung (DLCO), gallium citrate lung scanning and HRCT. A DLCO of
less than 80% of the predicted value has a sensitivity of up to 98% for PCP,
but the specificity is less than 50% and the measurement is not always
available. Although gallium scanning has a sensitivity of up to 100% for PCP in
patients with abnormal radiographs, it has never been prospectively studied in
patients with normal or equivocal radiographic findings. In addition, this
investigation requires a 48- to 72-hour delay in imaging, is not readily
available and has a high cost.

On the other hand, HRCT
is a widely available and noninvasive investigation for PCP. Patchy or diffuse
ground glass opacity is the most frequent finding. Other findings include
cystic changes (33%), centrilobular nodules (25%), lymphadenopathy (25%) and
pleural effusion (17%). HRCT has been found to be especially important in the
assessment of symptomatic patients with normal, equivocal or nonspecific
radiographs. In such cases, it shows high sensitivity (100%), specificity (86%)
and accuracy (90%) for PCP, using only the presence or absence of ground glass
opacity as the criterion for positivity.

The Arrow Shows The Dark Bronchus Relative to The Surrounding Lungs

Patchy ground opacity or mosaic attenuation, which is observed in up to 92% of the patients, can be easily identified on HRCT. However, subtle ground glass opacification, especially when bilateral and diffuse, may be difficult to diagnose. This is because of bilateral uniform increase in lung attenuation with absence of normal lung parenchyma for comparison. In such cases, the ‘dark bronchus’ appearance is a useful sign to identify diffuse ground glass opacity. This finding refers to the presence of air-filled bronchi appearing ‘too black’ relative to the surrounding lung parenchyma, which is filled with inflammatory alveolar exudates. This subtle finding may help in identification of patients with ‘possible PCP’ despite a normal or equivocal chest radiograph. Subsequently direct test for PCP (i.e., broncho-alveolar lavage) may be initiated for definitive diagnosis and treatment.Hence the importance of the ‘dark bronchus’ sign in the diagnosis of uniform, diffuse ground glass opacification on HRCT. This is especially useful in the presence of a normal chest radiograph and ‘near normal’ HRCT. HRCT offers an accurate and early diagnosis in patients with normal chest radiographs; it alters patient management and facilitates early therapy.Also Read